Firing waste wood in thermal power plants can lead to furnace wall corrosion due to corrosive elements such as chlorine, heavy metals, and alkali metals present in the fuel. This study investigates the influence of lead and chlorine on furnace wall corrosion of a low alloyed steel (16Mo3) and a nickel-based alloy (Alloy 625) during two field exposures using an air-cooled probe. Two two-week long test campaigns firing two different waste wood fuels (higher and lower lead and chlorine content) were carried out, exposing samples having metal temperatures in the interval 350-400 °C. The corrosion rates were determined using thickness loss measurements. The samples were examined using SEM/EDS/WDS and XRD techniques, to characterize the morphology and composition of the corrosion products. The findings suggest that higher lead and chlorine content in the fuel results in a higher corrosion rate for both materials; aggravated further above 370 °C. On 16Mo3 samples, iron oxides and chlorides, and chlorine-rich compounds are observed. Pits are observed on Alloy 625 samples, filled with nickel- and chromium-containing oxides mixed with corrosive species. The presence of lead compounds (e.g. lead molybdate) in connection to pits suggests active participation of lead in the corrosion process above 370 °C.